Process for stabilizing dairy products against age-thickening



United States Patent 3,008,840 PROCESS FOR STABILIZING DAIRY PRODUCTSAGAINST AGE-THICKENING Aubrey P. Stewart, .lr., Chicago, Ill., andRohertA. Johnson, San Jose, and Patricia T. Anderson, San Mateo, Calif.,assignors, by mesne assignments, to Hawthorn- Mellody, Inc., Chicago,11]., a corporation of Delaware No Drawing. Continuation of applicationSer. No. 484,583, Jan. 27, 1955. This application Oct. 22, 1958, Ser.No. 768,860

17 Claims. (Cl. 99-212) This invention relates to a new and improvedprocess for preparing sterilized dairy products, including whole andskimmed milk, evaporated milk, cream, and sterile concentrated milk,which has a. concentration greater or less than that normally existingin commercial evaporated milk. More particularly, this invention relatesto a method for treating dairy products, such as evaporated orconcentrated milk, after the same have been subjected tohightemperature, short-time sterilization, to prevent subsequentin-the-can gelation, or a-gethickening, or to defer such gelation to atime beyond the time span within which such products will have beennormally consumed.

This application is a continuation of our copending application SerialNo. 484,583, filed January 27, 1955, and now abandoned.

For many years it has been conventional practice to produce canned,sterilized milk, evaporated milk, cream, and other dairy products byfirst sealing the product in the can and then heating the sealed can andcontents in a retort under steam pressure. Although the cans wereusually rotated or shaken during the heating period, for the most partthe time and temperature conditions required to achieve sterilizationwere such that the resultant milk product had a strong cooked flavor anda brown color. Typical time and temperature conditions for thesterilization of milk and evapoarted milk by the agitationretort processare as follows: about 15 to 20 minutes to raise the temperature to 240F.; about 15 minutes to sterilize the milk at 240 F. to 245 F.; andabout 10 to 15 minutes to cool the milk to 70 to 80 F. Milk, cream andevaporated milk products sterilized by the above method, althoughpossessing a strong cooked flavor and brown color, are substantiallyfree from the tendency to age-thicken or gel.

In order to overcome the objectionable cooked flavor and brown colorcharacteristics of products sterilized according to such conventionalpractice, efforts have been made to employ what is known ashigh-temperature, shorttime sterilization. It is well known that, byemploying high temperatures and short holding times for sterilization,cooked flavor and brown color can be greatly minimized. In the past fewyears equipment has been developed which produces better agitation ofthe product sealed in the can and therefore enables a faster and moreuniform transfer of heat to the product. Such equipment enables in thecan sterilization to be conducted at a temperature of about 260 F. to265 F. with a holding time of 5 minutes or less. Another recentdevelopment is a method of canning known as aseptic canning. This methodenables the product to be pumped through high velocity heaters whichsterilize the milk at temperatures of 265 F. to 300 -F. with holdingtimes in the range of a few minutes to a few seconds. The sterilizedmilk is then quickly cooled in a previously sterilized cooler,aseptically canned by filling into previously sterilized cans, andsealed with previously sterilized lids.

Although high-temperature, short-time sterilization minimizes the degreeof cooked flavor and brown color imparted to the product, many dairyproducts sterilized under these conditions have a tendency toagethicken.

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This tendency is particularly marked in the case of evaporated milkwhere objectionable age-thickening occurs within a storage period ofthree to six months. Whole milk and cream will frequently exhibitobjectionable agethickening in nine months to one year.

With the process of the subject invention, evaporated milk can beproduced within a minimum of cooked flavor and brown color while beingresistive to age-thickening over a period of more than six months. Inlike manner, whole milk and cream, when treated in accordance with thesteps of the subject process, will have their periods of resistanceto'age-thickening materially enhanced.

This invention is based upon the discovery that the tendency towardage-thickening can be minimized by adjusting the salt balance of theproduct after high-temperature, short-time sterilization and prior tostorage. An essential object of the invention is to provide a method fortreating evaporated milk, and the like, after the same has beensubjected to high-temperature, short-time sterilization, to prevent orhinder age-thickening during storage.

A further object of the invention is to provide a method for treatingevaporated milk, 'and the like, after the same has been subjected tohigh-temperature, short-time sterilization, to both prevent or hinderage-thickening and undesirable insoluble salt precipitation duringstorage.

In carrying out this invention, as applied to evaporated milk, it isfirst desirable to prepare a concentrated milk which is stable towardsterilization heat treatment. While at certain seasons of the year it ispossible to merely pasteurize and evaporate the milk to obtain aconcentrate which will not heat coagulate during sterilization, most ofthe time special measures must be taken to stabilize the concentrate orelse coagulation will occur during sterilization. This stabilization maybe accomplished in several known ways, including the following: by theaddition of small amounts of disodium phosphate or sodium citrate; byforewarming the milk prior to evaporationor forewarming the concentrateafter evaporation but prior to sterilization; by utilizing ion-exchangetreatment of the concentrate prior to sterilization.

The method of adjustment of salt balance after sterilization, accordingto our invention, will depend, to a certain extent, on the manner inwhich the concentrate was prepared prior to sterilization as well as thesterilization temperature and time employed. However, we have discoveredthat in order to minimize age-thickening, it is necessary to increasethe concentration of divalent cations, Ca++ or Mg++, or decrease theconcentration of monovalent cations, Na+ or K": Furthermore, we havediscovered that this must be done in such a manner as not to causelater, during storage, the precipitation of insoluble calcium ormagnesium salts in the form of unsightly and objectionable crystals. Onemanner of accomplishing both of these ends is to employ sterileion-exchange treatment after the concentrate itself has been sterilized.

It should be understood that it is not practical to increase the Ca++ orMg++ ion concentration prior to sterilization since these cationsgreatly de-stabilize the concentrate in relation to ability of theconcentrate to be high-temperature, short-time sterilized without heatcoagulation. Similarly, decreasing the Na+ or K+ ion concentration priortosterilization renders the concentrate unshtable to heat. By utilizingour discovery, the age-thickening may be prevented or inhibited byadjusting the salt balance of the concentrate after sterilization, weare able to eliminate the problems connected with the prevention of heatcoagulation during sterilization.

In practice, we have prepared high-temperature, shorttime sterilizedevaporated milk, with a storage stability of more than six months, bythe following procedure, by way of example: whole milk was forewarmed toa temperature of F. for 20 minutes and then evaporated under vacuum to asolids content of 28 percent. To this concentrate there was added asmall amount of 30% -fat cream and water to give a final composition of26% total 1 solids of which 8.0% was butterfat. This concentrate wasthen sterilized by passage through a high velocity heater, which raisedthe temperature to 275 F., and through a holding tube which gave a 37seconds holding time. The sterilized concentrate was then passed througha previously sterilized pre-cooler and homogenized at a temperature of140 F. Following sterile homogenization, the product was cooled to 80 F.and filled into previously sterilized cans containing previouslysterilized ion-exchange material. The filled cans were then sealed withpreviously sterilized lids.

The ion-exchange material used in this example was prepared as follows:Chempro C-20, a strong acid cation polystyrene resin exchanger, wasreceived in the sodium state from the manufacturer thereof, the ChemicalProcess Company of Redwood City, California. To be assured ofreactivation, the resin was first acid regenerated by the addition of 10volumes of 5% hydrochloric acid to one volume of wet, tamped resin. Themixture was held for 5 minutes with stirring and decanted. The resin wasagain treated with volumes of 5% hydrochloric acid, held for 5 minuteswith stirring and decanted. The resin was then treated with 10 volumesof hydrochloric acid, the mixture held for 5 minutes with stirring, andthen decanted. The resin was washed with previously boiled distilledwater until no acid was noted in the wash water by litmus paper test.

The previously acid treated resin was then regenerated to the sodiumform by adding 10 volumes of 10% sodium chloride solution to one volumeof wet, tamped resin, the mixture held 5 minutes with stirring, anddecanted. The resin was again treated with 10 volumes of 10% sodiumchloride solution, held 5 minutes with stirring,

and decanted. The resin was then washed with previously boiled distilledwater until no salt taste was noted in the wash water.

After being washed free of excess sodium chloride, the resin was thenput through a cycle of mixed calciumsodium regeneration, accomplished bytreating each 100 ml. (tamped volume) of the sodium-regenerated resin'with 15.1 grams of anhydrous calcium chloride dissolved in 100 ml. ofdistilled water. The mixture was held 5 minutes with stirring, decanted,washed with distilled water, dried, and sterilized. This resin was thenused in treatment of the sterilized evaporated milk in the amount ofapproximately 0.4% by weight.

The particular technique of using a mixed calciumsodium regeneratedresin is desirable for operational convenience, since it was found thatthis type of resin bed would not coagulate the evaporated milkcontacting the resin or produce a slime which would make a continuousflow column unusable after a short period of operation. However, withspecial precaution, it is possible to accomplish the desired results ofsalt balance adjustment by a number of procedures. Among theseprocedures which we have found to be effective in various degrees are:use of a conventional column type ion-exchange process; use of amodified column type and employing backwash techniques; and ion-exchangetreatment of a portion of the sterilized evaporated milk followed by ametering of this portion back into the main stream.

Among the resins used were Chempro C-20 and Duolite CS-lOl forcation-exchange, and Duolite A-4 and Duolite A-30 for an ion exchange.These resins were all produced by the Chemical Process Company ofRedwood City, California.

Although the chemistry of salt balance adjustment after sterilization isnot definitely known in relation to inhibiting age-thickening, orgelation, it is thought that the inhibiting efiect=is caused by properaddition of divalent cations, represented by Ca++ and Mg, in such amanner as to react with the proteins and yet not form insoluble calciumphosphate and/or calcium citrate. It is also our discovery that the mostelfective salt balance adjustment of evaporated milk after sterilizationis one which also partrally removes monovalent ions, represented by Na+and K+. Thus, in the example given, not only is Ca added,

but also the ion-exchange treatment serves to remove Na+ and K+ from theevaporated milk.

It is to be understood that the ion-exchange treatment and salt balanceadjustment above-described, which is etfective to minimizeage-thickening, is not the type of salt balance adjustment described inthe literature for stabilizing evaporated milk to heat coagulation. Infact, the subject salt balance adjustment is opposite in nature, sincein adjusting the salt balance to stabilize against heat coagulation thepractice is to'add Na+ (e.g. sodium citrate or sodium phosphate) orremove Ca+ If a given concentrate has been stabilized against heatcoagulation prior to sterilization by way of addition of sodium cit-rateor sodium phosphate, it is advisable, according to the subject process,to utilize an amount of ion-exchange material in excess of normal aftersterilization to remove a suflicient amount of the added Na+ to preventor hinder age-thickening.

Also, we have found that a sterile solution of calcium chloride may beadded to evaporated milk, after sterilization and prior to storage, toprevent or hinder age-thickening. However, while the addition of calciumions in this manner eifectively hindered age-thickening, there was aprecipitation of insoluble calcium salts in the milk on storage. Suchprecipitation results in down-grading of the commercial product,although the product is still salable. The method of the presentinvention is to so add calcium ion, such as in the preferred way abovedisclosed, to prevent such precipitation.

It is our intention, by placing the ion-exchange material in the canprior to filling with sterile evaporated milk, to simulate a possiblyideal condition wherein the ion-exchange material may be included in theresin lining of the can, as manufactured.

Within the concept and scope of our invention is ionexchange treatmentof evaporated milk which is hightemperature, short-time sterilized by inthe can" sterilization equipment. Thus, the ion-exchange material in thecan may be coated with a substance which prevents the aforementionedundesirable salt balance adjustment prior and during sterilization, butwhich dissolves to allow the salt balance adjustment to take place aftersterili' zation in order to age-thickening. Preparation of the can linerin such a manner as to allow the salt balance adjustment reaction totake place very slowly, so that the major portion of the reaction occursafter sterilization, would permit the elimination of the slowlydissolvable protective coating over the ion-exchange material.

It has been found that the lower the temperature and the longer theholding time used in sterilization, the less the extent of salt balanceadjustment required after sterilization. However, our invention isapplicable where the sterilization conditions range in temperature fromsubstantially 260 F. to 310 F., with holding times of substantially fiveminutes to less than one second.

While the above description of the subject process has been made withspecific reference to evaporated milk, the process is to be understoodas being applicable to other milk products where age-thickeningtendencies exist on storage after high-temperature, short-timesterilization. While specific examples of carrying out the process havebeen described in detail, it is to be understood that these are onlyillustrative, since the process is capable of modification to takecare-of different conditions and requirements. Such variations arecontemplated as come within the spirit of the invention and the scope ofthe appended claims.

What is claimed is:

1. A method tor producing a. sterile evaporated milk with a minimum ofcooked flavor and stabilized against age-thickening which comprisesstabilizing the milk against heat coagulation by reducing the reactivedivalent to monovalent ion ratio thereof, evaporating the milk toincrease the solids ratio thereof to the desired degree, subjecting theevaporated milk to a temperature between 260 F. and 310 F. for a periodof from five minutes to one second to sterilize said milk,'cooling thesterilized evaporated milk, increasing the reactive divalent to.

monovalent ion ratio of the cooled evaporated milk to a value in excessof such ratio after said stabilization againstheat coagulation bybringing said milk into reactive relationship with an ion-exchangematerial, thereby stabilizing the milk against age-thickgening, andsealing the milk in a container under sterile conditions.

2. The method of claim 1, in which said evaporated milk is sterilized byrapidly heating the same to a temperature of approximately 275 F. andholding the milk at said temperature for approximately one-half minute.

3. The method of claim 1, in which said milk is stabilized against heatcoagulation by forewarrning the same to an elevated temperature.

4. The method of claim 1 including the step of bringing said cooledevaporated milk into reactive relationship with ion-exchange material byproviding such material in said container.

5. The method of claim 4 including the step of lining at least a portionof the interior of the container with said ion-exchange material.

6. A method for producing a sterile evaporated milk with a minimum ofcooked flavor and stabilized against age-thickening which comprisesstabilizing the milk against heat coagulation by reducing the reactivedivalent to monovalent ion ratio thereof, evaporating the milk toincrease the solids ratio thereof to the desired degree, heating theevaporated milk to a temperature between 260 F. and 310 F. for a periodof from five minutes to one second to sterilize said milk, cooling thesterilized evaporated milk, increasing the reactive divalent tomonovalent ion ratio of the cooled, sterilized evaporated milk to avalue in excess of such ratio after said stabilization against heatcoagulation, said increase in divalent ion ratio preventing theformation of objectionable crystals and stabilizing the milk againstage-thickening, and sealing the milk in a container under sterileconditions.

7. The method of claim 6 in which said milk is stabilized againstage-thickening by treating the same to increase the concentrationtherein of reactive calcium ions and decrease the concentration thereinof monovalent alkaline metal ions.

8. A method for producing a sterile milk product stabilized againstage-thickening which comprises sterilizing the product, increasing thereactive divalent to monovalent ion ratio of the sterilized product to avalue in excess of such ratio prior to sterilization of the product,said increase in divalent ion ratio preventing the formation ofobjectionable crystals and stabilizing the product againstage-thickening, and sealing the product in a container under sterileconditions.

9. The method of claim 8 in which said dairy product 6 is milk and inwhich the product is sterilized by heating the same to a temperature inexcess of 260 F. for a short period and then cooling the same prior tosaid stabilization against age-thickening.

10. The method of claim 8 in which the reactive divalent to monovalention ratio is increased by bringing the sterilized product into reactiverelationship with an ion-exchange material.

11. A method for producing a sterile evaporated milk with a minimum ofcooked flavor and stabilized against age-thickening which comprisesstabilizing the milk against heat coagulation by reducing the reactivedivalent to monovalent ion ratio thereof, evaporating the milk toincrease the solids ratio thereof to the desired degree, sterilizing themilk, and then sealing the sterilized evaporated milk in a containerunder sterile conditions, said container having therein an ion-exchangematerial capable of increasing the reactive divalent calcium ion tomonovalent alkaline metal ion ratio of the milk to a value in excess ofsuch ratio after said stabilization against heat coagulation, therebystabilizing the milk against age-thickening.

12. A method for producing a sterile milk product stabilized againstage-thickening which comprises sterilizin the product and sealing theproduct in a container, said container having therein a material capableof increasing the divalent to monovalent ion ratio of the product to avalue in exceess of such ratio prior to sterilization of the product.

13. A method forproducing a sterile evaporated milk stabilized againstage-thickening which comprises evaporating milk to increase the solidsratio thereof, heating the evaporated milk to high temperatures for ashort time to sterilize the evaporated milk, cooling the sterilizedmilk, and then increasing the then existing divalent ,,to' monovalention ratio of the cooled, evaporated and sterilized milk, said increasein divalent ion ratio preventing the formation of objectionablecrystals.

14. An aseptically canned sterile evaporated milk made by the process ofclaim 1.

15. An aseptically canned sterile evaporated milk made by theprocess ofclaim 6. I

16. An aseptically canned sterile evaporated milk made by the process ofclaim 11.

17. An aseptically canned sterile milk product made by the process ofclaim 12.

References Cited in the file of this patent UNITED STATES PATENTS2,064,110 Hall Dec. 15, 1936 2,087,962 Ball July 27, 1937 2,135,054Schwartz Nov. 1, 1938 2,360,033 Baumann Oct. 10, 1944 2,374,427 FearApr. 24, 1945 2,503,866 Chrysler et a1. Apr. 11, 1950 2,511,825 MyersJune 13, 195.0 2,570,231 Hansen Oct. 9, 1951 2,541,568 Baur Feb. 13,1951

1. A METHOD FOR PRODUCING A STERILE EVAPORATED MILK WITH A MINIMUM OFCOOKED FLAVOR AND STABILIZED AGAINST AGE-THICKENING WHICH COMPRISESSTABILIZING THE MILK AGAINST HEAT COAGULATION BY REDUCING THE REACTIVEDIVALENT TO MONOVALENT ION RATIO THEREOF, EVAPORATING THE MILK TOINCREASE THE SOLIDS RATIO THEREOF, TO THE DESIRED DEGREE, SUBJECTING THEEVAPORATED MILK TO A TEMPERATURE BETWEEN 260*F. AND 310*F. FOR A PERIODOF FROM FIVE MINUTES TO ONE SECOND TO STERILIZE SAID MILK, COOLING THESTERILIZED EVAPORATED MILK, INCREASING THE REACTIVE DIVALENT TOMONOVALENT ION RATIO OF THE COOLED EVAPORATED MILK TO A VALUE IN EXCESSOF SUCH RATIO AFTER SAID STBILIZATION AGAINST HEAT COAGULATION BYBRINGING SAID MILK INTO REACTIVE RELATIONSHIP WITTH AN ION-EXCHANGEMATERIAL, THEREBY STABILIZING THE MILK AGAINST AGE-THICKGENING, ANDSEALING THE MILK IN A CONTAINER UNDER STERILE CONDITIONS.